Backwater valves are installed in liquid flow lines to prevent a reversing of the flow. One example of a flow line in which a backwater valve would be used is in a building sewer line going from a private residence to a main sewer line. The backwater valve prevents raw sewage from flowing back into the private residence should the main sewer line overflow.
An example of a backwater valve is U.S. Pat. No. 1,606,396 which issued to Blom in 1926. The Blom reference discloses an upper horizontal section of pipe with a downwardly turned elbow and a lower horizontal section of pipe with an upwardly turned elbow. The downwardly turned elbow and the upwardly turned elbow are connected with a horizontal annular valve seat positioned therebetween. A normally pendant float valve is suspended from the valve seat. When water rises in the lower horizontal section of pipe the float valve floats into engagement with the annular valve seat to restrict the flow.
There are number of disadvantages with backwater valves constructed in accordance with the teachings of Blom. The float valve is subject to a build up of hair, toilet paper and other debris. This debris prevents the float valve from fully closing. If the float valve does not fully close the valve merely serves to restrict rather than eliminate the backflow and raw sewage will seep into the residence. Blow recognized this potential problem in the design and provided for both a cover secured by bolts above the valve seat and a plug provided near the hinge of the float valve to allow access for cleaning purposes. The backwater valves are underground installations which are difficult to access for the purpose of maintenance, so as a practical matter it would not be discovered that the valve was in need of cleaning until it failed to work.
In order to address these problems axial flow backwater valve mechanisms have been developed that have indirect valve closure mechanisms. An example is U.S. Pat. No. 4,503,881 which has a gate pivoted from above. The gate is attached to a float mechanism in an antechamber. Liquid entering the antechamber causes the float mechanism to rise. As the float mechanism rises the gate is closed. Backwater valves with indirect closure mechanisms are more complex and as a consequence are more expensive.